1. Field of the Invention
The present invention relates generally to a bipolar transistor and a method of manufacturing the same, and in particular to a bipolar transistor used for a high-frequency operation and a method of manufacturing the same.
2. Background Art
A transistor formed of GaAs, which is a III-V compound semiconductor, has characteristics such as high operating frequency, low noise, high output, high gain, low operating voltage, high operation efficiency, low power consumption, etc, which are superior to those of a silicon transistor. Because of such characteristics, GaAs field effect transistors (FETs) and heterojunction bipolar transistors (HBTs) have already been used as devices for mobile communication. In such GaAs transistors, HBTs are superior to FETs as amplifiers and oscillators since they have higher gain and higher breakdown voltage.
Recently, a new transistor, in which a GaAs HBT is combined with a material such as Ge, InGaAs, etc., has been proposed, as disclosed in the Japanese Journal of Applied Physics, 1991, Vol. 30, pp. 1,659–1,663.
Conventional GaAs HBTs have a problem in that operating voltages thereof are high.
It is obvious that a decrease in control voltage would result in a decrease in power consumption. As a method of decreasing control voltage, the use of a material having a lower band gap energy than GaAs as a material of a base layer has been proposed. However, no material has sufficiently solved the aforementioned problem.
Specifically, the aforementioned article (Japanese Journal of Applied Physics) discloses a transistor using Ge as a material of a base layer. The band gap of Ge (about 0.7 eV) is smaller than the band gap of GaAs (about 1.4 eV). In addition, the degree of lattice mismatching between Ge and GaAs is small. Accordingly, the aforementioned article proposes a transistor having a low control voltage, which is achieved by combining a GaAs HBT with a Ge base layer. However, Ge has a problem in that the degree of diffusion is great. Because of this diffusion problem, no GaAs HBT with a Ge base layer has actually been commercialized. Other transistors proposed use InGaAsN, GaAsSb, and InGaAs as a material for a base layer. However, although InGaAsN can be lattice-matched with a GaAs substrate, crystal-growth thereof is difficult to achieve, the etching thereof is difficult, and a theoretical ideal voltage decrease effect is only 0.25 eV. Furthermore, although GaAsSb has an advantage of forming a type-II heterojunction, the degree of lattice mismatching is great, and the crystal growth is difficult to achieve. Although InGaAs is commonly used for forming an emitter contact layer, it is difficult to use this for forming a base layer since the degree of lattice mismatching is great.